This application deals with the regulation of sterol and bile acid biosynthesis in the rat and other laboratory animals. Specifically, it is proposed to investigate: 1) The in vivo regulation of bile acid biosynthesis by the circulating bile acid pool, by cholesterol and other sterols, and by drugs, hormones and diet. 2) The properties and subcellular distribution of enzymes likely to be rate-limiting in sterol and bile acid biosynthesis namely 3-hydroxy-3-methylglutaryl CoA reductase (EC 1.1.1.34), cholesterol 7 alpha-hydroxylase, 3-hydroxy-3-methylglutaryl CoA synthase (EC 4.1.3.5), 3-hydroxy-3-methylglutaryl CoA lyase (EC 4.1.3.4), 3-hydroxy-3-methylglutaryl CoA deacylase (EC 3.1.2.5) and acetoacetyl-CoA deacylase. 3) The activities of the rate-limiting enzymes of sterol and bile acid biosynthesis as affected by the circulating bile acid pool, by cholesterol and other sterols, and by drugs, hormones and diet. 4) The metabolism of 5 alpha-cholestan-3 beta-ol with emphasis on the biosynthetic pathway, turnover in blood and tissues, and its possible regulatory effect on adrenal cholesterol synthesis.